Key to this project is the identification of the underlying mechanisms that cause the lymphopenia and reduced host defense that accompanies zinc deficiency (ZD) in humans and animal. The hypothesis to be tested is that ZD heightens apoptosis, reduces cycling and rate of production of precursor lymphocytes in the marrow and thymus. Conversely, the first line of defense, the phagocytic or myeloid cells, appear to be afforded some protection against ZD perhaps to provide minimal immune defense. Using the young adult mouse the following will be our objectives. 1. Lymphopoiesis: The effects of ZD over time on the phenotypic distribution, cell cycle status and rate of production and replenishment of precursor cells in the marrow will be assessed. 2. Myelopoiesis - Hematopoiesis: The impact of ZD on the stem cell, progenitor, erythroid and myeloid compartments will be monitored and the effect (if any) of ZD on myelopoiesis determined. Because there is a significant increase in neutrophils in the blood of ZD mice, it will be important to determine if they retain their phagocytic and killing capacity and if their half-life is altered. 3. Role of Apoptosis: Experiments will be extended that indicate apoptosis is significantly increased among thymocytes from ZD mice. The glucocorticoid antagonist RU38486 will be implanted to reduce the impact of the steroids produced during ZD to potentially open the way for drug and/or cytokine therapy in malnourished subjects. 4. Zinc - Induced Apoptosis: Finally, it will be shown that physiologically relevant levels of zinc (nM) can induce apoptosis in precursor cells being a new role for zinc. Human leukocytes, testicular cells, kidney cells, hepatocytes, etc., will be tested for propensity to undergo zinc-induced death. Whether the zinc death cascade includes classical components such as cell condensation, lipid inversion, activation of caspases, DNA fragmentation and induction of death genes will be determined. Transgenic mice over-expressing bcl-2 in their B-cell compartment will be used to determine if this important protooncogene acts as a regulatory point for the Zn-death pathway. Collectively, these experiments will provide the first detailed picture of the effect of a nutritional deficiency on the composition, function and regulation of the marrow and the role Zn can play in modulation of apoptosis.